Rats and mice are the species most frequently used for cellular and biochemical studies of plasticity, but only a few studies have examined developmentally regulated visual plasticity in these species. Here we report a study of the critical period for monocular deprivation in Long-Evans rats in which visual pattern sweep evoked potentials (sweep VEP) was used. Successful recording of sweep VEPs depended on establishing a stable light plane of anesthesia. We found a mixture of halothane and NO2 to be suitable. During a single trial lasting 10 s, anesthetized rats (n = 28) viewed a sinusoidal contrast grating (spatial frequency of 0.13 cycles/deg) that reversed phase at 3 Hz. During the trial, the grating contrast increased logarithmically from 1 to 70%. Extracellular recording pipettes were placed bilaterally in layers II/III of the binocular regions of primary visual cortex. Stimulating the right and left eye on alternate trials, sweep VEP amplitudes were collected for 30 trials from each eye. In monocularly deprived animals, the right eyelid had been sutured for 5 days before recording. Age at suture varied from P19 to P86. In 12 of 13 rats sutured between P19 and P50, the crossed response from the deprived eye was smaller than the crossed response from the nondeprived eye. The same relation prevailed for the uncrossed responses in 11 of 13 animals. There was no significant monocular deprivation effect in animals sutured between P55 and P86 (n = 9). Dark rearing until approximately P90 followed by 5 days of eyelid suture resulted in a strong monocular deprivation effect in both crossed and uncrossed pathways (n = 3). There was little effect of dark rearing alone on the size the sweep VEPs (n = 3). The critical period reported here lasts at least 2 wk longer than reported for rats by Fagliolini et al. and for mice by Gordon and Stryker. Both previous studies used single unit recording rather than the sweep VEP method.